The effectiveness of heat guns is predicated upon the ability of the combustion products to entrain and propel vast amounts of the surrounding air. Two factors have been found to enhance this process: 1) The speed of the combustion products to be as high as possible and 2) the combustor outlet to be in the shape of a slot in order to maximize the gas/air interface and create a fan shaped heat output pattern.
The speed of the combustion products is a function of the pressure recovery of the jet pump which is used to aspirate the combustion air by the expansion of the gaseous fuel. The performance of the jet pump is thus linked directly with the effectiveness of the heat gun.
One measure to improve the performance of prior art jet pumps has been to lengthen the diffusor to achieve maximum pressure recovery. One drawback of pushing the diffusor to its limits is the attendant tendency for flow separation and pressure fluctuation. The periodic flow separation occurs spontaneously, even in a perfectly draft-free room, but are exacerbated by any disturbance: by moving the heat gun about, by air drafts and even by sound. The result is an uneven flow, noisy combustion, bad emissions and performance fluctuations.
Another measure to improve the performance of prior art jet pumps has been to use multiple nozzles in place of a single nozzle. These efforts have aimed to arrange the nozzles to shorten the mixing process and minimize friction losses in the mixing duct of the jet pump.
The fan shaped pattern has the advantage of spreading the heat evenly over a wide area. The heated area is a long, narrow zone in line with the combustor slot which the operator sweeps over the object to cover the whole area.
The orientation of the slot relative to the handle of the heat gun is usually a matter of personal preference but in some instances also of practical significance. When shrinking a plastic bag over a pallet for instance, it is important to first shrink the bottom of the bag all around to prevent it from riding up. A horizontal orientation of the slot is most efficient for this operation. Subsequently, when shrinking the sides of the bag, a vertical orientation is more effective. Thus it is desirable to change the orientation of the slot easily and quickly.
One commercially available heat gun employs a screw with a wing head to fasten the cylindrical combustor inlet to the body of the heat gun so that the operator can adjust its orientation without tools. This arrangement however is awkward in practice since the mounting screw has to be loosened and tightened every time the slot orientation is changed. If the operator neglects to tighten the screw, he runs the risk of loosing it.
Another need that arises in practice is to extend the length of the heat gun to heat objects which are out of reach. This situation occurs for instance when shrink wrapping tall pallet loads or big boats. In the past this has been accomplished by extension tubes. The extension tube ducts the combustible mixture from the jet pump to the combustor as well as providing an electrical lead and ground from the ignitor to the spark plug. The installation is particularly cumbersome. First the fasteners holding the combustor have to be removed, the spark plug lead disconnected and the combustor taken off. Then the process has to be repeated twice in the reverse order, once to attach the extension to the gun, and again to mount the combustor to the extension. Disassembly is an equally complicated process. An added problem arises in keeping the second set of fasteners from getting lost.
A serious ignition problem arises with the extension if the ignition lead is carried inside the extension tube. After operating the gun a few times the spark grows progressively weaker until it is unable to light off the gun. The only solution to this problem in the past has been to mount the ignition lead outside the extension tube. This arrangement is costly and makes the ignition lead vulnerable to damage in use.